In the bitcoin transaction version 2, there are additional two bytes (hex: "0001") between transaction version (hex: "02000000"), and number of inputs (hex: "01"). What it is?

Here's an example https://btc-explorer.com/tx/4f692e7fb63256e4e741b952935e929ad545838d3697c5ee3f044a250c45fb4a

transaction hex: "020000000001010000000000000000000000000000000000000000000000000000000000000000ffffffff4b031a0608048187145b622f4254432e434f4d2ffabe6d6dec50b9ca7f0e6731e89a2516f3df5fd7d65462ed2e68df7bf8e375232b7b28c701000000000000006200de217cb2aaea00000000ffffffff0212b0bf4a000000001976a91478ce48f88c94df3762da89dc8498205373a8ce6f88ac0000000000000000266a24aa21a9ed89dbbf72d51df862d19e0baa551d87e69b65a452496ec9d8203de66c754c30330120000000000000000000000000000000000000000000000000000000000000000000000000"

2 Answers 2


That is not something specific to version 2 transactions.

What you are looking at are the marker and flag bytes for a segwit transaction. Segwit transactions have a slightly different transaction format which is specified in BIP 144.


You might be interested in my article The Bitcoin Script Language. A part of that is to go into details of the transaction format. Especially helpful is this graphic I've created:

enter image description here

To answer the question directly: The first one is called "marker", the second one is called "flag". BIP 144 gives more details:

  • Rationale for not using just a single 0x00 byte as marker: that would lead to empty transactions (no inputs, no outputs, which are used in some tests) to be interpreted as new serialized data.
  • Rationale for the 0x01 flag byte in between: this will allow us to easily add more extra non-committed data to transactions (like txouts being spent, ...). It can be interpreted as a bitvector.

See BIP 141

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